1. Field of the Invention
The present invention relates generally to active metal electrochemical devices. More particularly, this invention relates to active metal (e.g., lithium) fuel cells made possible by active metal electrode structures having ionically conductive membranes for protection of the active metal from deleterious reaction with air, moisture and other fuel cell components, methods for their fabrication and applications for their use.
2. Description of Related Art
In recent years, much attention has been given to hydrogen and/or fossil fuel based fuel cells. A fuel cell is an electrochemical device that continuously changes the chemical energy of a fuel (e.g., hydrogen) and an oxidant (e.g., oxygen in air or water) directly to electrical energy, without combustion. Fuel atoms give up their electrons in the process. Like a battery a fuel cell has electrodes and electrolyte. However, while a battery stores energy, a fuel cell generates it from fuel and oxidant supplied to the electrodes during operation. In a hydrogen fuel cell, oxygen is typically supplied to the oxygen electrode (cathode; electrode to which cations migrate) from ambient air, and the hydrogen fuel is supplied to the fuel electrode (anode) either from a pressurized cylinder or from a metal hydride forming alloy. Fossil fuel based fuel cell systems extract the required hydrogen from hydrocarbons, such as methane or methanol.
Active metals are highly reactive in ambient conditions and can benefit from a barrier layer when used as electrodes. They are generally alkali metals such (e.g., lithium, sodium or potassium), alkaline earth metals (e.g., calcium or magnesium), and/or certain transitional metals (e.g., zinc), and/or alloys of two or more of these. The following active metals may be used: alkali metals (e.g., Li, Na, K), alkaline earth metals (e.g., Ca, Mg, Ba), or binary or ternary alkali metal alloys with Ca, Mg, Sn, Ag, Zn, Bi, Al, Cd, Ga, In. Preferred alloys include lithium aluminum alloys, lithium silicon alloys, lithium tin alloys, lithium silver alloys, and sodium lead alloys (e.g., Na4Pb). A preferred active metal electrode is composed of lithium.
The low equivalent weight of alkali metals, such as lithium, render them particularly attractive as electrode materials. Lithium provides greater energy per volume than the traditional hydrogen fuel or fossil fuel fuel cell standards. However, it has not previously been possible to leverage the advantages of lithium and other alkali or other active metals in fuel cells. Previously, there was no way to isolate the highly reactive anode alkali metal fuel from the cathode oxidant while maintaining a path for the alkali metal ions.